The large storage capacities and relative low costs of CD-ROMS and DVDs have created an even greater demand for still larger and cheaper optical storage media. Holographic memories have been proposed to supersede the optical disc as a high-capacity digital storage medium. The high density and speed of the holographic memory comes from three-dimensional recording and from the simultaneous readout of an entire packet of data at one time. The principal advantages of holographic memory are a higher information density (1011 bits or more per square centimeter), a short random access time (˜100 microseconds and less), and a high information transmission rate (109 bit/sec).
In holographic recording, a light beam from a coherent monochromatic or multispectral source (e.g., a laser) is split into a reference beam and an object beam. The object beam is passed through a spatial light modulator (SLM) and then into a storage medium. The SLM forms a matrix of shutter (in the binary case) or, more generally, a matrix of photocells modulating the light intensity that represents a packet of data. The object beam passes through the SLM which acts to modulate the object beam with the binary information being displayed on the SLM. The modulated object beam is then directed to one point on the storage medium by a beam processor where it intersects with the reference beam to create a hologram representing the packet of data.
An optical system consisting of lenses and mirrors is used to precisely direct the optical beam encoded with the packet of data to the particular spatially addressed area of the storage medium. Optimum use of the capacity of a thick storage medium is realized by spatial and angular multiplexing. In spatial multiplexing, a set of packets is stored in the storage medium shaped into a plane as an array of spatially separated and regularly arranged subholograms by varying the beam target in the x-axis and y-axis of the plane. Each subhologram is formed at a point in the storage medium with the rectangular coordinates representing the respective packet address as recorded in the storage medium. In angular multiplexing, recording is carried out by keeping the x- and y-coordinates the same while changing the irradiation angle of the reference beam in the storage medium. By repeatedly incrementing the irradiation angle, a plurality of packets of information is recorded as a set of subholograms at the same x- and y-spatial location.
Previous techniques for recording information in a highly multiplexed volume holographic memory, and for reading the information out are limited in memory capacity. In particular, data storage using magnetic, magneto optic, and optic technology is limited to a capacity that most likely will not go beyond 1 terabyte per unit. Most of the current advanced commercial products are in the range of 100 gigabytes. The need for terabyte and petabyte mass storage is becoming evident for hospital applications, for picture storage, mapping (the whole word map on a tape), defense, Internet, database, meteorology, and so forth.
It is therefore an object of the present invention is to provide an apparatus for reading a diffractive memory capable of a storage capacity ranging from terabyte to petabyte mass tape and disk storage.
It is a further object of the present invention is to provide a massive petabyte tape storage system. It is still a further object of the present invention to provide a massive terabyte disk storage system.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and ensuing description.